1. Field
Disclosed herein is a method for recovering gold hydrometallurgically from a sulphidic concentrate, particularly one containing arsenopyrite and/or pyrite. The concentrate is first subjected to leaching with a concentrated solution of alkali chloride and copper (II) chloride, by means of which the copper minerals and some of the gold in the concentrate are made to dissolve. Elemental sulphur and precipitated iron and arsenic compounds are separated from the leaching residue using physical separation methods, whereby the first intermediate is obtained, which contains gold-bearing sulphide minerals and gangue minerals as well as the gold that remains undissolved. The free gold that remains undissolved is separated by means of gravity separation methods. After gravity separation, additional comminution is carried out, after which the sulphide minerals are broken down and the gold-containing solution or residue is routed to the concentrate leaching circuit.
2. Description of Related Art
Copper concentrates contain variable amounts of gold. In smelting plant processes gold is generally recovered with a high yield via anodic sludge treatment processes. In hydrometallurgical copper processes the recovery of gold from concentrates causes a specific problem. Gold recovery in process alternatives using sulphate-based leaching is usually based on cyanide leaching of leach residue, whereby however the elemental sulphur formed in copper leaching disrupts the cyanide leaching of gold. In chloride-based copper processes, both the gold bound to copper minerals and the free gold dissolve to a large extent, but the gold bound to pyrites and silicates as fine inclusions or to sulphide minerals as what is termed invisible gold, remains mainly undissolved. Invisible (submicroscopic) gold is inside the mineral particles as very fine inclusions or in the mineral lattice. Some of the coarse free gold contained in the concentrate also remains undissolved due to too short a retention time.
In refractory gold concentrates, the proportion of copper and other base metals is usually small. The recovery of gold by cyanide leaching alone does not succeed with concentrates in which the gold is refractory or submicroscopic. One example of this kind of concentrate is a concentrate containing arsenopyrite and/or pyrite. Gold recovery from such concentrates to requires the almost total decomposition of the minerals containing the gold. If cyanide leaching is used, the concentrate requires pretreatment, such as roasting, bioleaching or oxidising pressure leaching.
Outokumpu Oyj has developed a hydrometallurgical copper recovery process, the HydroCopper™ process, which is described for example in U.S. Pat. No. 6,007,600. According to this, the copper concentrate is leached in atmospheric conditions into a concentrated alkali chloride solution using divalent copper as oxidant. The leaching of gold in connection with the HydroCopper process is described in for example WO patent application 03/091463. According to this, gold dissolves during copper concentrate leaching as a chloride complex and is recovered from the solution using activated carbon. However, if gold appears in a difficult form e.g. in pyrite and/or in silicate minerals, it cannot be leached with the method described in the above-mentioned WO application.
Patent application WO 2004/059018 describes a gold recovery process, in which refractory gold-containing concentrate such as arsenopyrite or pyrite is treated in a halide environment in atmospheric conditions. The arsenopyrite and pyrite lattice is broken down using chemical oxidation. Oxygen is used to form a soluble oxidant in the form of divalent copper or trivalent iron. With divalent copper, arsenopyrite decomposes and forms arsenic acid, divalent iron, sulphur and monovalent copper. Iron and copper are oxidised with oxygen to a higher valence. The trivalent iron thus formed reacts further with the arsenic acid forming ferric arsenate (FeAsO4). The decomposition of pyrite occurs in the same way by means of divalent copper, so that sulphuric acid and divalent iron (Fe2+) are formed. Divalent iron is oxidised to trivalent and monovalent copper to divalent by means of oxygen. Iron is precipitated as hematite and the solution is neutralised by feeding limestone into it, so that gypsum (CaSO4) is precipitated out. If carbon is included, the concentrate is roasted after the leaching stages. Gold dissolves from the pyrite as a chloride complex and is recovered using activated carbon.
Refractory gold ores can be treated with the method according to WO patent application 2004/059018, but the disadvantage is that all the sulphur generated from both arsenopyrite leaching and copper minerals leaching has to be oxidised to sulphate. Arsenic first enters the solution from which it is is precipitated as ferric arsenate, but the sulphur generated in arsenopyrite leaching proceeds with the solids to the subsequent leaching stage, where it is oxidised to sulphate. In this case there is a great need for oxidation and likewise the need for neutralisation increases considerably, which weakens the economy of the process significantly. The entire amount of concentrate in the process is ground very fine, up to 80% smaller than 6-10 μm, so that the demand for grinding capacity is large, and the grinding energy consumption is high while at the same time sludging problems increase and both solids and liquid separation stages become more complicated.
U.S. Pat. No. 6,315,812 describes the Platsol™ process, in which sulphide minerals or smelting matte are treated with oxidising pressure leaching in a solution containing chloride and sulphate.
In the Platsol process all the sulphur in the sulphide phase is oxidised to sulphate, whereupon the need for neutralisation increases greatly, reducing the process economy. The use of chloride in autoclave conditions leads to expensive investments due to the corrosion question etc.
U.S. Pat. No. 6,461,577 describes a two-stage bioleaching method for leaching sulphides that contain arsenic. Gold is recovered from the resulting solution by cyanide leaching.
Bioleaching as the only leaching method for the total amount of concentrate is fairly slow. The disadvantages of the bioleaching method are the difficult solubility of chalcopyrite and the oxidation of the entire amount of concentrate to sulphate, where the need for neutralisation is large. In addition, cyanide is used to leach gold, which poses a risk for the environment.